1. Field of the Invention
The present invention relates to a controller for a switching power converter that drives a BJT (Bipolar Junction Transistor) switch.
2. Description of the Related Arts
Achieving low cost and high performance at the same time is a challenge in designing low-power AC/DC power supplies. Compared with conventional secondary-side feedback switching power supplies, primary-side feedback switching power converters offer great cost savings and enhanced reliability by removing components such as opto-couplers and output current sensing resistors. However, conventional controller ICs (Integrated Circuits) used in primary-side feedback switching power converters often use the input line voltage as a parameter in controlling the operation of the switching power converter for output voltage regulation. Thus, these conventional power supply controller ICs typically have a Vin pin that senses the instantaneous line voltage (input voltage) as rectified by rectifier circuitry, adding an extra pin to the controller IC. In addition, sensing the line voltage also requires an analog-to-digital converter (ADC) and its associated circuitry for converting the analog input voltage to digital values that can be used by the digital controller ICs, control logic internal to the controller IC, and a resistive voltage divider external to the controller IC for sampling and feeding the line voltage information to the Vin pin. Such additional pin and extra circuitry add to the die size and manufacturing cost of the controller IC.
Meanwhile, using a BJT as the power switch in the switching power supply has several benefits, including lower cost than power MOSFET's, less EMI (Electro-Magnetic Interference) noise, and eliminating costly and lossy snubber circuitry for low-power designs of 2-3 W. To achieve high performance including high efficiency, however, the driver design for BJT's is more challenging and complicated.